[colinlm]

To be fair, starting with the Apple IIe, the $C019 softswitch is a good, light way of syncing to VBL. Doing it with the mouse IRQ has an advantage when one requires a mouse, though: it works the same way on the whole line of Apple II computers, starting with the II+.

Without the mouse IRQ, if one wants to support the whole line of Apple II computers, one has to vapor lock on the II+, remember that the IIgs's $C019 high bit means the inverse than the IIe, and the //c requires more trickery around it. (cf https://github.com/cc65/cc65/blob/master/libsrc/apple2/waitv...)